This invention relates to a construction for fastening moving blades to a rotor wheel of a turbine, more particularly to a construction for fastening Titanium alloy moving blades to a rotor wheel made of different material in Young's modulus from the moving blades.
Moving blades of a steam turbine are fastened to a rotor wheel, with dovetails of the moving blades being inserted into recesses provided on the rotor wheel. Recently, the steam turbine is being made larger in capacity, which accompanys more elongated moving blades in the last stage. In a conventional steam turbine, as a material for moving blades, 12% Cr-alloy is used. Where moving blades made of such an alloy are made longer for example more than 40 inches (1016 mm), and used at a rotational speed of 3600 r.p.m., there is a danger such that the moving blades are broken by a centrifugal force. Therefore, for such long moving blades, it is preferable to use a material as strong as the 12% Cr-alloy and small in specific gravity. As such a material, a Titanium alloy is known. Young's modulus of the titanium alloy is very small as compared with that of an alloy used for a turbine rotor. Therefore, when the moving blades of the Titanium alloy are fastened to the turbine rotor made of an alloy which is different from the Titanium alloy with a conventional fastening construction, as shown in FIGS. 1 and 2 of Japanese Laying-open of Patent Application No. 50-139205 (1975), the dimension of which are determined by taking into consideration only strength of both the rotor material and the moving blade material, serious problems may take place. Namely, in the fastening construction of the moving blade and the rotor, deformation of the moving blade due to various force applied on the moving blade differs from that of the rotor, therefore, ununiform stresses are induced in the moving blade and the rotor, and concentrated stresses are induced in a portion of the moving blade or the rotor wheel. Such a fastening construction has danger such as cracks may take place in a portion larger in stress, that is, a portion at which the concentrated stresses are induced.